The present invention disclosed herein relates to electric devices and methods of manufacturing the same, and more particularly, to thermoelectric devices and methods of manufacturing the same.
Recently, research into thermoelectric devices has been actively conducted as interests in clean energy have grown. A thermoelectric device may convert thermal energy into electric energy, or conversely, may generate a temperature difference by applying electric energy thereto.
A ZT value (thermoelectric figure of merit value) is used as an index that determines thermoelectric efficiency of the thermoelectric device. The ZT value is proportional to the square of the Seebeck coefficient and electric conductivity, and is inversely proportional to thermal conductivity. The ZT value may be defined as a unique property for each corresponding material. For example, with respect to metal, a value of the Seebeck coefficient is very low at a few μV/K. The electric conductivity and the thermal conductivity are in a proportional relationship by the Wiedemann-Franz law. This means that, with respect to metal, heat transfer mostly occurs by free charges such as electrons or holes. Therefore, with respect to metal, the realization of low thermal conductivity that is essentially required for the thermoelectric device may be very difficult. Also, the improvement of the ZT value using metal may be virtually impossible.
However, with respect to a semiconductor, since its charge concentration may be freely adjusted, heat transfer by free charges may be appropriately controlled. A major mediator of the heat transfer in the semiconductor is a lattice, and a phonon is lattice vibration which is quantized and described as waves. Therefore, if the concentration of the free charges in the semiconductor is appropriately adjusted to minimize the heat transfer by the free charges and inhibit the propagation of phonons, thermal conductivity may be rapidly decreased.
As commercialized materials for a thermoelectric device, Bi2Te3 is used at room temperature and SiGe is used at a high temperature. A ZT value of Bi2Te3 is 0.7 at room temperature and is a maximum of 0.9 at 120° C. A ZT value of SiGe is about 0.1 at room temperature and is a maximum of 0.9 at 900° C.
Research into thermoelectric devices based on silicon, a basic material of the semiconductor industry, has attracted attention. Since silcon has a ZT value of 0.01 due to its very high thermal conductivity of 150 W/m·K, it has been recognized that utilization of silicon as a thermoelectric device may be difficult. However, with respect to silicon nanowires grown by chemical vapor deposition (CVD), it has recently been reported that thermal conductivity may be decreased to 0.01 times or less, and accordingly, a ZT value is close to 1.